Open Valve Spring Pressure
There are a few terms used
in determining which pushrod would best fit your needs that would be beneficial
for you to understand. One major factor for determining the correct tube
diameter and wall thickness is “open valve spring pressure”. This is the
pressure exerted against the spring retainer when the valve is opened as far as
the cam grind allows. The required open valve spring pressure is usually
specified in the cam instructions but knowing the actual “true” open valve spring
pressure is the most accurate information. Other factors affecting this load
number are valve train geometry (angles of lifter to pushrod and pushrod to
rocker arm), rocker arm ratios, whether it’s a naturally aspirated engine and,
of course, what diameter you have room for.
Radius 101
Determining which ball
and/or cup you require is also quite important. We will quiz you to help us
determine what ends you need on your pushrods. For many of the common engines,
we know the radius of the stock pushrods but when rebuilding and/or replacing
components with aftermarket parts, it’s never safe to assume. The most accurate
way of determining the correct radius (curve) that is needed for each end of
the pushrod is by using a set of radius gauges. Measure the radius of the puck
(pocket) of the lifter and the ball or cup of the rocker arm that will be used.
Most machine shops have radius gauges but most hobbyists do not. In some cases
the best way to get the correct ends for your project is to send us samples of
the parts being used and we will measure them.
Determining Length
This can be one of the
hardest measurements to determine. Because of varying valve train geometry and
design, determining the correct pushrod length is a science unto itself. First,
let’s tackle some terminology. There’s “overall length” which is the length
measured from extreme end to end. This is an easy length to measure if you have
a pushrod the correct length then all you need is a caliper long enough and you
just take a physical measurement. However, an “overall” measurement on a cup
style pushrod may not be what you want as the depth of the cup is a factor. For
that style pushrod, the “effective” length is more accurate. That length is
from where the contact point on both ends of the pushrod is (Bottom of the
inside of the cup to the lifter end). You can take an overall measurement then
measure how deep the cup is and subtract that dimension to come up with
effective or place a ball bearing the proper size inside the cup then measure
over the bearing and subtract the full diameter to get the same result.